Active matrix type liquid crystal display devices generally include a substrate having thin film transistors (hereinafter, also referred to as “TFT”) which are provided to respective ones of the pixels and serve as switching elements (hereinafter, referred to as “TFT substrate”), a counter substrate having a color filter and other components, a liquid crystal layer provided between the TFT substrate and the counter substrate, and a pair of electrodes for applying a voltage across the liquid crystal layer.
For active matrix type liquid crystal display devices, various operation modes corresponding to their uses have been proposed and adopted. Examples of the operation mode include TN (Twisted Nematic) mode, VA (Vertical Alignment) mode, IPS (In-Plane-Switching) mode, and FFS (Fringe Field Switching) mode.
Among these modes, the TN mode and the VA mode are vertical electric field modes in which an electric field is applied to liquid crystal molecules by a pair of electrodes that are arranged so as to sandwich the liquid crystal layer. The IPS mode and the FFS mode are transverse electric field modes in which a pair of electrodes are provided on one substrate, and an electric field is applied to liquid crystal molecules in a direction parallel to a surface of the substrate (transverse direction). In the transverse electric field modes, the liquid crystal molecules do not rise from the substrate, and therefore, there is an advantage that a wide viewing angle can be realized as compared with the vertical electric field modes.
FFS mode liquid crystal display devices are disclosed in, for example, Patent Document 1 and other documents. In TFT substrates used in these devices, a common electrode and a pixel electrode are provided above TFTs with an insulating film provided therebetween. Of these electrodes, typically, one provided on the liquid crystal layer side (e.g., pixel electrode) has an opening in the form of a slit. Thus, an electric field produced is represented by a line of electric force extending from the pixel electrode and passing through the liquid crystal layer and then through the slit opening before reaching the common electrode. This electric field includes a component transverse to the liquid crystal layer, and therefore, the liquid crystal molecules aligned in the horizontal direction can be rotated in a plane.
Meanwhile, forming the active layer of the TFT using an oxide semiconductor instead of a silicon semiconductor has been proposed. Such a TFT is referred to as “oxide semiconductor TFT”. For example, Patent Document 2 discloses an active matrix type liquid crystal display device in which oxide semiconductor TFTs are used as the switching elements.
The oxide semiconductor has a higher mobility than amorphous silicon. Therefore, oxide semiconductor TFTs are capable of higher speed operation than amorphous silicon TFTs. Also, oxide semiconductor films can advantageously be formed through a simple and convenient process as compared with polycrystalline silicon films.
The size of the oxide semiconductor TFT is small. Thus, when this is used in a display device, the pixel aperture ratio can be improved as compared with a case where a conventional TFT is used. Therefore, brighter display can be achieved. Alternatively, the brightness of the backlight may be reduced, and the power consumption can be reduced.
Further, since the off-leak current of the oxide semiconductor TFT is very small, a single gate TFT can be employed rather than a conventional double-gate configuration, whereby size reduction can be realized. Also, the retained voltage in an OFF period is appropriately maintained for a relatively long period of time. Thus, it is possible to employ an operation mode in which the operation frequency is decreased according to the conditions of use, and the power consumption can be reduced without causing display errors, such as flicker.